The invention relates to a process for continuously preparing heat-vulcanizable silicone compositions.
A heat-vulcanizable silicone composition comprises a high viscosity silicone polymer, an inorganic reinforcing filler and various additives that aid processing or impart desired final properties to the composition. A vulcanizing agent can be added and the composition heat-cured to fabricate silicone rubber moldings such as gaskets, medical tubing and computer keypads.
Typically, the heat-vulcanizable silicone composition is produced by kneading a high-viscosity polydiorganosiloxane, the inorganic filler and additives by means of a batch kneading machine such as a high intensity Banbury mixer or a low intensity double arm dough mixer. In this process, polydiorganosiloxane, inorganic filler, treating agents and additives are batch mixed until desired properties are obtained. In Kasahara et al., U.S. Pat. No. 5,198,171, a preconcentrate of polydiorganosiloxane, inorganic filler and treating agents is formed by a high speed mechanical shearing mixer. The resulting premix is further compounded in a same-direction double screw extruder. The premix is formed in a first step wherein a diorganopolysiloxane having a viscosity at 25° C. of 1×105 cP or more, an inorganic filler and a treating agent are mixed in a high speed mechanical shearing machine to provide a flowable particulate mixture in which each ingredient is present in a substantially uniform, finely dispersed state. The flowable particulate mixture is then fed at a constant feed rate into a kneading and extruding machine that has two screws rotating in the same direction.
A batch process requires long mixing times and large amounts of energy. Non-homogeneous shear and extensional stress across a commercial sized batch can result in non-uniform size distribution of filler that results in variations in properties. Batches processed at different times may be characterized by different physical properties. The batch process is labor, energy and capital intensive and produces materials of only marginal consistency.
In Hamada et al., U.S. Pat. No. 5,409,978, a preconcentrate of polydiorganosiloxane, inorganic filler and treating agents is formed at a temperature in the range of about 200° C. to 300° C. in a co-rotating continuous double screw extruder. The preconcentrate is then compounded and heat treated at 150° C. to 300° C. in a counter-rotating, double screw extruder. However, a process that requires two extruders is expensive and requires significant processing area.
However with these processes, throughput is limited. There is a need for a process that provides improved throughput and which can be adapted as a low cost process that can efficiently utilize a single extruder to continuously and consistently produce a full range of both low viscosity and high viscosity silicone elastomers from filler, additive and polymer.